And now I am going off on a tangent about Dorothy's motives in The Wonderful Wizard of Oz, which were, you remember, to go home.
The precise statement that triggered this was that Dorothy's motive was not strong enough. Which is certainly arguable. She lacked what T. S. Eliot termed an objective correlative: an external focus for her emotions, to convince us how deeply she felt. (Better than the movie, where she purportedly learned that she needed to go home, but still immediately asked how to get home, and was sent to the wizard. Phooey. You want to give her a character arc, you got change her at the beginning to make her need to learn. Then, you didn't give her anything that would really learn it.)
But the desire for home is only one of many things that can start the story off. The abduction of a parent, or sibling, or beloved, or child, or the theft of a precious object. Etc. The problem is that if you do not want the kidnap victim, for instance, to be merely a MacGuffin for the quest, you need characterization and stuff to provide the objective correlative. Which takes up time and space. And delays the action. And quite possibly makes the genre unclear.
Bridging conflict, where the hero and the kidnap victim are involved working with a different problem before the abduction, is one solution, but doesn't always work best. (If they already have problems, for instance, it may be a problem if the abduction has to be a surprise to the main character.) But wrestling the info into the story is always interesting.
Especially for portal fantasies, since the character's old life is on the other side of the portal.